1. Field of the Invention
The present invention relates to a lens barrel which holds a zoom lens consisting of multiple lens groups, to a photographic apparatus which takes photographs by capturing light from a subject entering through the zoom lens held by the lens barrel, and to an optical apparatus which has a lens consisting of multiple lens groups.
2. Description of the Related Art
Recently, digital cameras have been spreading rapidly, and increasingly higher image quality is demanded together with smaller size and thinner profiles. A thin card-size digital camera equipped with a lens barrel which holds a zoom lens consisting of multiple lens groups has been proposed (see Non-patent Document 1: Internet URL—http://www.business-ijP/sentan/jusyou/2003/pentax/) and introduced commercially as one of digital cameras which meet the above needs. This camera is capable of high image quality zooming using optical zoom whereas earlier thin digital cameras use an electronic zoom function for magnification.
One of the trends in user needs is to have higher-powered optical zoom capabilities while achieving smaller size and thinner profiles.
Patent Document 1 (Japanese Patent Laid-Open No. 2003-295031) proposes a technique for collapsing a lens barrel equipped with a zoom lens in a thin camera body using an ingenious method for forming cam grooves.
An internal configuration of the lens barrel disclosed in Patent Document 1 is described below.
FIGS. 1, 2, and 3 are sectional views taken along the optical axis of the lens barrel mounted in a digital camera. Of these, FIGS. 1 and 2 show the lens as it is extended. According to Patent Document 1, FIG. 1 is a diagram showing a telephoto end while FIG. 2 is a diagram showing a wide-angle end. FIG. 3 is a diagram showing the lens barrel as it is collapsed. FIG. 4 is a developed view illustrating cam grooves used to extend and collapse the lens barrel from/into the camera body.
A configuration of a lens barrel 100 is described with reference to FIGS. 1 to 4.
The lens barrel 100 holds a four-group zoom lens composed of a first lens group 210, second lens group 220, third lens group 230, and fourth lens group 240. Of the four lens groups, the second lens group 220 is moved along the optical axis for adjustment of focal distance while the fourth lens group 240 serving as a focus lens is moved along the optical axis for focus adjustment.
The first lens group 210 is held in an inner tube 110. The inner tube 110 is equipped with cam pins 111, which are engaged with cam grooves 121 (see FIG. 4) formed in the inner surface of an outer tube 120. Three cam pins 111 are installed on the outer wall of the inner tube 110 at unequal intervals and three cam grooves 121 are formed on the inner wall of the outer tube 120 at unequal intervals (see FIG. 4) to engage with the cam pins 111. Consequently, when rotation of a zoom motor 270 is transmitted to a gear 124 installed on the inner wall of the outer tube 120 via a coupling gear 271 (see FIG. 2), rotating the outer tube 120, the first lens group 210 extends together with the inner tube 110 following the shape of the cam grooves 121.
The second lens group 220 is held by a lens group holding frame 221, on whose circumference three cam pins 222 are installed at unequal intervals. The cam pins 222 are engaged with respective cam grooves 122 formed in the inner wall of the outer tube 120 (see FIG. 4). A guide rod 1132 is passed through a through-hole 221a made in the lens group holding frame 221 of the second lens group 220. Along with rotation of the outer tube 120, the second lens group 220 moves along the optical axis, being guided by the guide rod 1132. The guide rod 1132 is supported by a tip support 1132a while a guide rod 1133 which guides the third lens group 230 is supported by another tip support 1133a. The tip supports 1132a and 1133a support the respective guide rods 1132 and 1133 and are also used as members which support the inner tube 110. The tip supports 1132a and 1133a are equipped with an intermediate frame 1101 and a retainer ring 1102. The intermediate frame 1101 is inserted slidably along the inner wall of the inner tube 110 and the retainer ring 1102 is installed at the rear end of the inner tube 110 to prevent the intermediate frame 1101 from moving backward. The intermediate frame 1101 and retainer ring 1102 are equipped with respective spring pegs 1101a and 1102b. A spring 1103 is bridged between the spring pegs to restrict the movement of the intermediate frame 1101 by urging the intermediate frame 1101 forward along the sliding surface so that the intermediate frame 1101 will not move backward when it is extended together with the inner tube 110.
Furthermore, a cam groove 123 (see FIG. 4) is formed between the cam grooves 121 and 122 in the outer tube 120 to engage with a cam pin (not shown) installed on a lens group holding frame 130 of the third lens group 230. Consequently, as the outer tube 120 rotates by receiving the driving force of the zoom motor 270 via the coupling gear 271 and gear 124 (see FIG. 2), the third lens group 230 moves along the optical axis following the shape of the cam groove 123. Incidentally, a shutter unit 131 is linked to the lens group holding frame 130 which holds the third lens group 230.
A through-hole 131a is made in the lens group holding frame 130 as in the case of the lens group holding frame 221. The guide rod 1132 is passed through the through-hole 131a. Furthermore, the guide rod 1132 is also passed through a through-hole 141a made in a lens group holding frame 140 which holds the fourth lens group 240 described later. In this way, this example employs a configuration in which the second lens group 220, third lens group 230, and fourth lens group 240 are guided by the common guide rod 1132 to avoid misalignment of optical axes.
Extension operation of the lens barrel 100 with this configuration is described in detail with reference to FIG. 4.
When the outer tube 120 is rotated by the zoom motor 270, the inner tube 110 extends from a collapsed state to an extended position (B-side end of the area indicated by symbol A) following the shape of the cam grooves 121 (area indicated by symbol A) and held at the extended position (area indicated by symbol B). Until the outer tube 120 is held at the extended position, the second lens group 220 moves along the area indicated by symbol C following the shape of the cam grooves 122 and reaches the end of the area indicated by symbol C when the inner tube 110 is extended to the extended position. As a zoom switch (not shown) is operated at this time, the second lens group 220 enters the area indicated by symbol D, and moves to the end of the area indicated by symbol D if the zoom switch continues to be operated. On the other hand, as the inner tube 110 rotates, the third lens group 230 leaves a collapsed position, moves along the cam groove 123 through an extension area (area indicated by symbol E) and reaches an extended position (intersection of areas indicated by symbols E and F). It remains held at the extended position (area indicated by symbol F) even if the zoom switch is operated.
In this way, by arranging cam grooves ingeniously in the inner wall of the outer tube 120, it is possible to move the first lens group, second lens group 220, and third lens group 230 among the four lens groups along the optical axis following the shape of the cam grooves by the rotation of the single tube 120 and move the second lens group along the optical axis by the operation of the zoom switch, and thereby do zooming.
In the lens barrel 100 shown in FIGS. 1 to 3, the fourth lens group 240 at the tail of the four lens groups composing the zoom lens is used as a focus lens. The zoom lens held in the lens barrel 100 has a high zoom ratio, and consequently the fourth lens group 240 acting as the focus lens must have a relatively long travel distance. Thus, in this example, a column screw 1131 (see FIG. 1) as long as the long travel distance is installed along the optical axis, a nut 141b is fastened to the lens group holding frame 140 which holds the fourth lens group 240, and the column screw 1131 is screwed into the nut 141b for accurate focus adjustment.
Focus is adjusted as the column screw 1131 is rotated by rotational driving force of a focus motor (not shown) transmitted via a gear train (not shown) and a lens group holding frame 141 moves along the optical axis by the distance equivalent to the rotation of the column screw 1131, being guided by the column screw 1131 and guide rods 1132 and 1133. When an image taking lens starts to catch a subject, focus is adjusted by generating image data by means of an image pickup device 280, detecting a focus position based on the image data, and moving the fourth lens group 240 acting as the focus lens to the focus position through rotation of the column screw.
After focus adjustment, when a shutter button (not shown) is pressed, the shutter unit 131 provided in the lens group holding frame 130 which holds the third lens group 230 is operated in, synchronization with full depression of the shutter button to take a photograph. Consequently, light from the subject passes through the first lens group 210, second lens group 220, third lens group 230, and fourth lens group 240 (focus lens) and forms an image on a light-receiving surface of the image pickup device 280, which generates an image signal which represents the subject image formed on the light-receiving surface.
Even if attempts are made to further reduce the length of the lens barrel, since the lens barrel shown in FIGS. 1 to 4 consists of two tubes, it is not possible to reduce the length sufficiently. It is conceivable to reduce the length of the lens barrel by increasing the number of tubes of the lens barrel to three or four. In that case, however, to arrange lens groups of the collapsed lens barrel along the optical axis, it is necessary to build a safety factor into the length of the lens barrel to allow adjacent lens groups to stay clear of each other. Besides, for high-powered zooming, it is also necessary to increase the length of the extended lens barrel to allow for a longer adjustable distance between lens groups composing the zoom lens.
Thus, due to the increase in the length of the extended lens barrel, the length of the collapsed lens barrel cannot be decreased as it ought to be.